Leukemia is a cancer that starts in blood stem cells in bone marrow, making it especially hard to localize as it carries throughout the body once in the bloodstream.

Try reviewing this:

https://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0CDYQFjAB&url=http%3A%2F%2Fwww.cancer.ca%2Fen%2Fcancer-information%2Fcancer-type%2Fleukemia%2Foverview%2F%3Fregion%3Dab&ei=oVJsU8vSJdLpoATwp4CABA&usg=AFQjCNGtr3ZgwBGpfU8DXS18qfuYXzTRWg&sig2=wKt0t4m36WZOJRnJTtXiFA&bvm=bv.66330100,d.cGU

and/or this:

https://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CCoQFjAA&url=http%3A%2F%2Fen.wikipedia.org%2Fwiki%2FLeukemia&ei=oVJsU8vSJdLpoATwp4CABA&usg=AFQjCNFjFd0Eqviqj3OWuWYXT_gHT9VK6A&sig2=38bfkGKKyyokjP-ul-5s3A&bvm=bv.66330100,d.cGU